FIG. 1 shows an overview of known air conditioning equipment comprising a compressor 14 and an external oil separator 12, these having a connection 17 to heat exchangers (not shown). During a compression process, gas enters the compressor 14 under suction 40. Oil is injected into the compressor 14 to improve efficiency and to provide cooling of the compressor 14. A gas and oil mixture is created in the compressor 14 which is delivered via a first inlet pathway 13 to the oil separator 12. Once separated, the gas is delivered via the connection 17 to the heat exchangers and the oil is delivered via a third pathway 16 back to the compressor 14. The quantity of oil allowed to enter a cooling system such as an air conditioning system must be kept to a minimum if heat exchanger efficiency is to be maintained.
Screw compressors have become increasingly popular for refrigeration and air conditioning applications in recent years. Their high reliability, small size and weight for a given capacity, make these compressors ideal for use in packaged chiller units. Environmental issues are increasingly important and thus also efficient operation of these chillers.
FIG. 2 shows an example of a single screw compressor comprising a single main rotor 100 with two meshing gate rotors 110, 115. The single main rotor 100 has a number of helical flutes 105, which are cut with a globoid (or hour glass) shape to the roots of these threads flutes. The flutes 105 have a relatively large cross section at an input end 120 and a significantly smaller cross section at a discharge end 125.
Suction gas enters the flutes 105 at the large openings at the input ends 120, in a generally axial direction with respect to the main rotor 100. The gas is then sealed into the flutes 105 by the gate rotors 110, 115 and casing (not shown) as the rotor assembly 100, 110, 115 rotates, the discharge ends 125 of the flutes 105 normally being closed by the casing. Continued rotation causes the teeth of the gate rotors 110, 115 to progress along the flutes 105 causing a reduction in volume and thus an increase in pressure. The compressor is so designed that when the desired pressure increase has been reached the flute opens to a discharge port in the casing and continued rotation causes the refrigerant gas to be driven out through the discharge port. The design allows for this compression process to be mirrored on both sides of the main rotor 100 by the use of two gate rotors 110, 115.
FIG. 2 shows a compression process in three different rotational positions. In a first position, shown to the left in FIG. 1, a gas-filled flute 105 has a relatively large volume, indicated by a dotted area. As the input end 120 is sealed by a tooth of a gate rotor 115 which begins to move along the gas-filled flute 105 during rotation of the rotor assembly 100, 110, 115, the volume of the gas-filled flute 105 reduces, as shown in the middle of FIG. 1. The volume of the gas-filled flute 105 reaches a minimum just as its discharge end 125 comes level with a discharge port (not shown) in the casing. This last rotational position is shown to the right in FIG. 1. The gas expands as it is released through the discharge port. This process is repeated for each consecutive flute 105.